Various air fuel ratio detecting devices have been proposed which make use of characteristics of the oxygen concentration cell action and the oxygen ion pumping action of zirconia as disclosed, for example, in Japanese patent laid-open No. 56-130649. However, the accuracy of such conventional air fuel ratio detecting devices are not sufficiently high as to allow the air fuel ratio detecting devices to be used for detecting a stoichiometric air fuel ratio, such as, for example, for a ternary catalyzer, due to its structure.
Besides, in the case of an O.sub.2 sensor used as a conventional type stoichiometric air fuel ratio detecting device, deviations from the stoichiometric air fuel ratio cannot be readily discriminated, and accordingly the width of the fluctuations in the feedback control of the air fuel ratio is comparatively great, which will cause variation in the output power of the engine or in the number of engine revolutions during idling of the internal combustion engine.
An air fuel ratio detecting device is disclosed in Japanese patent application No. 60-262982 corresponding to U.S. patent application Ser. No. 933,850 wherein a sensor cell detects the difference between the concentration of oxygen in the exhaust gas and the concentration of oxygen in a reference gas so as to develop a corresponding electrical signal. In response to the electrical signal, a controlling means produces and delivers an electrical controlling signal to a pump cell so that the pump cell may cause, in response to the electrical controlling signal received, movement of oxygen ions until the level of the air fuel ratio, and the time at which the stoichiometric air fuel ratio is reached, are detected by first and second detecting means from the information regarding the air fuel ratio as transmitted from the controlling means to the pump cell.
However, where an air fuel ratio detecting device of the aforementioned type is employed, there is the problem that the degree of accuracy of the device may be low due to a possible error between products or due to the fact that the device may deteriorate with time due to extensive use.